Project Summary/Abstract Cardiometabolic disease as result of metabolic syndrome continues to be a major health burden worldwide, regardless of the numerous available therapeutic strategies. Obesity and diabetes impact cardiovascular health leading to heart failure, stroke, coronary vascular disease and other indices like fatty liver disease due to perturbed metabolic homeostasis. Derangements in systemic energy homeostasis are the result of metabolic dysfunction within individual tissues as well as in defective communication between tissues. Understanding metabolic coordination within a particular tissue and throughout the body will lead to new and improved therapeutic avenues. We recently uncovered a novel class of secreted micropeptides expressed in the brain, among which is a protein we call B03. Mice with genetic deletion of B03 have elevated plasma lipids and reduced respiratory exchange rates, leading to our hypothesis that B03 has a role in metabolic homeostasis by communicating nutritional status between neurons. This proposal will determine if the micropeptide B03 is required to maintain whole body energy balance through actions within the brain. First we will characterize the metabolic outcomes of genetic deletion of B03 in the whole animal. We will subject B03 knockout animals to metabolic stress paradigms including diet induced obesity as well as severe hypoglycemia. These animals will then undergo a series of assessments using state-of-the-art techniques including metabolic cages to measure energy expenditure, neuroendocrine assays, and gene expression measurements in liver and adipose. We will next determine the exact cellular identity of neurons expressing B03 using histochemical techniques. Finally, we will delete or overexpresss B03 only in neurons using either cre-recombinase or adeno-associated virus (AAV) tools and assess the animals as stated above to determine the requirement of the brain in mediating the effects of B03 on metabolic homeostasis. Taken together, these studies will test the hypothesis that B03 is a novel neuropeptide that contributes to metabolic homeostasis by acting within the brain in response to nutritional challenge.